In endoscopy and related fields, such as borescopes and dental scopes, the complete optical system is thought of as consisting of four basic and separate optical functions. Those functions are, in sequence of the direction of the travelling light, as follows:                1. an objective which forms the first image of an object under surveillance,        2. a field lens which images the pupil of the objective onto the next image transfer lens,        3. an image transfer lens which reimages the first image onto the next field lens. The pupil and image transfer steps are repeated as often as is needed to obtain a desired tube length, and        4. a focussing lens which presents the final image to a sensor, like a person's eye, a CCD camera, or a photographic film.        
This approach is the classical approach, and it is appropriate for the following reasons:                1. The design of the optical system is broken up into parts with single and clearly defined and separate functions, functions to each of which an optical designer may bring considerable experience, and        2. The light transfer capacity and information transfer capacity of an endoscope is at a maximum when the optical power is concentrated at the image planes and pupil planes. The expedience of this approach is brought out by numerous U.S. patents on endoscopes which consistently treat the objective, the relay system, and the eyepiece as separate parts of the total system.        
The disadvantage of treating the different optical components as separate entities is that the distribution of the optical powers is very uneven and that certain aberrations are naturally at a maximum, like astigmatism, field curvature, and chromatic aberrations. The correction of these aberrations require relatively short radii. These short radii are difficult to fabricate, require tight tolerances, and they are therefore the main contributors to the considerable cost of the fabrication of an endoscope. A truly inexpensive endoscope, sufficiently inexpensive to be offered as a disposable item, is presently not practical with conventional designs.